Prevention of metal hydride decomposition



United States Patent Int. C1. 0611) 6/34 US. Cl. 21--60.5 4 Claims ABSTRACT OF THE DISCLOSURE Protection against decomposition of alkali metal hydride containing liquids by coating a surface thereof with a layer of liquid parafinic hydrocarbon which is substantially immiscible and non-reactive with the alkali metal hydride liquid, has a boiling point at least 20 C. higher than the temperature of the alkali metal hydride containing liquid and has a melting point below the temperature of the alkali metal hydride containing liquid.

This invention relates to a method of preventing the decomposition of metal hydride. It more particularly refers to the prevention of decomposition of liquid mixtures of alkali hydroxides and metal hydrides.

Metal descaling has become an important industrial process. It has been found that one particularly good descalant is a solution of a metal hydride, particularly an alkali metal hydride, in an eutectic or nearly eutectic mixture of metal hydroxides, particularly alkali metal hydroxides. Such descaling solutions are known to the art, see German patent application D 44,165 which corresponds to US. application Ser. No. 451,671, filed Apr. 15, 1965 now abandoned.

These solutions have been used to descale iron, steel and most non-ferrous metals in an efiicient manner. It is unfortunate however that the use of a solution of a metal hydride in a metal hydroxide eutectic or near eutectic for descaling or perhaps any other purpose sufiers from disadvantages having nothing whatever to do with the utility of such solutions as descalants or anything else.

The problem is that metal hydrides are quite reactive with the water content of the air such that the metal hydride decomposes and thus becomes unsuited to the use for which it or its solution was intended. Further, one of the products of decomposition is hydrogen gas which, as is well known, is quite flammable and is explosive in certain proportions in admixture with air. Thus the problem is three-fold: how to prevent metal hydride decomposition; how to prevent hydrogen ignition; and how to prevent explosion of hydrogen-air mixtures.

Various and sundry methods have been conceived to solve these related probelms. Some of the solutions suggested to solve these problems include mechanically covering the vat, trough or other vessel in which the metal hydride solution is maintained, which cover could conceiva'bly be of the same material as is the vessel or could be of another solid material hingedly or removably aflixed to the containing vessel. This proposal has not been partion-larly successful since the cover must be opened each time an article to be descaled or a finished descaled article is inserted into or removed from, respectively the vessel. Upon each opening a certain amount of air is, of necessity, permitted access to the metal hydride solution whereby such hydride can be decomposed or hydrogen decomposition product which was produced during the last previous access is given access to the air whereby an explosive or at least inflammable situation is potentially created.

3,468,621 Patented Sept. 23, 1969 It can be conceived that the surface of the metal hydride solution in metal hydroxide can be constantly swept with an inert gas such as nitrogen or a rare gas, but it must be appreciated that this would be an extremely expensive procedure to add to what is now a relatively inexpensive process whereby it could be debated as to whether it would be cheaper to permit the metal hydride to decompose or to blanket and sweep the solution with inert gas to prevent the decomposition of the metal hydride.

It is thus apparent that no economically practical, workable solution to the problems presented has as yet been found.

It is therefore an object of this invention to provide a method of preventing the decomposition of metal, particularly alkali metal, hydrides.

It is another object of this invention to provide a method of preventing such decomposition at elevated temperatures in a liquid metal hydroxide solution of such metal hydride.

Other and additional objects of this invention will become apparent from a consideration of this entire specification, including the claims appended hereto.

In accord with and fulfilling these objects, this invention resides, in one of its broadest aspects, in providing a liquid coating for a descaling liquid soultion of metal hydride.

Although reference has hereinbefore been made to solutions of metal hydrides in mixtures, particularly eutectic mixtures of metal hydroxides, and although this is one of the preferred embodiments of this invention, the method of this invention is applicable to prevention of decomposition of metal hydrides in general where such metal hydrides have the potentiality of being exposed to air or oxygen. With this in mind, this invention will be further described with respect to the embodiment thereof where metal hydride is dissolved in liquid mixtures of metal hydroxides.

In carrying out this invention, it is necessary for the liquid coating to have certain attributes. Such coating must be substantially immiscible and non-reactive with respect to the metal hydride or other components of the liquid it is coated upon. It must be a liquid at the temperatures of operation, must have a relatively low vapor pressure and flammability at these temperatures and must not decompose to any appreciable extent at these temperatures.

One class of materials, that has been found to be eminently suited to use in this invention possessing substantially all of the attributes required and set forth above, is relatively high molecular weight hydrocarbons, particularly paraflinic materials. These saturated hydrocarbons may be relatively low molecular weight olefin polymers or they may be naturally occurring or synthetic paratfin oils. Exemplary of the naturally occurring oils useful in this invention are crude oil fractions having melting points of about -50 to C. The commercial product Bayol marketed by Esso Standard Oil Company is exemplary of paratfins suited to use in this invention. Also suited are products of Fischer-Tropsch synthesis using cobalt catalysts in the known way. Oligomers and low molecular weight polymers of ethylene having molecular weights of about 3000 to 8000 are suited to use in this invention. Exemplary of the polyethylenes referred to are those marketed by BASF as Lupolen N and by US. Industries as Epolene.

Paraflins, according to this invention, should have boiling points at least about 20 C. higher than the hydride solution operating temperatures and should have melting points at least about 5 C. below such operating tempera ture.

The following examples are given by way of illustration of the invention and are in no way limiting thereon.

3 Example 1 An eutectic melt of sodium hydroxide mixed with potassium hydroxide was prepared containing 2% by weight sodium hydride. About eight (8) tons of this melt were disposed in a vessel such that the surface area of the melt was about five square meters. The bath temperature was brought to 230 C. and held there uncovered for about 48 hours after which time the sodium hydride content of the melt was measured to be 1% by weight.

Example 2 Example 1 was repeated except that the surface of the melt was coated with about 5 millimeters of a white oil having no components which distilled below 300 C. No loss of metal hydride could be determined at the end of the 48 hour period.

What is claimed is:

1. The method of preventing the decomposition of the alkali hydride content of a melt comprising a mixture of alkali metal hydride and alkali metal hydroxide, which comprises coating said melt on the surface thereof which would otherwise be exposed to air, with a liquid layer of a parafiinic hydrocarbon which is substantially immiscible and non-reactive with said alkali metal hydride containing melt, has a boiling point at least about 20 C. higher than the temperature of said alkali metal hydride containing melt and has a melting point below the temperature of said alkali metal hydride containing bath.

2. The method claimed in claim 1, wherein said paraffinic hydrocarbon has a melting point of about to C.

3. The method as claimed in claim 1, wherein said alkali metal is at least one member selected from the group consisting of sodium, potassium and lithium.

4. The method as claimed in claim 1, wherein said melt is a mixture together with sodium hydride of a substantially eutectic mixture of sodium hydroxide and potassium hydroxide.

References Cited UNITED STATES PATENTS 2/ 1956 Carter 1342 X 8/1959 Banus et al. 23-204 US. Cl. X.R. 

